The trimming (adjustment) of resistors is a widely used procedure in the manufacture of microelectronics and electronic components, and in common design of user circuits, especially where precision calibration is desired. In principle, one trims the resistor until an observable local or global circuit parameter reaches a desired value. Resistor trimming is widespread in both manufacturing of a variety of components and instruments, and in the user community.
The design of any high-precision analog electrical circuit must entail careful consideration of temperature variation. This is especially true when the circuit involves thermally mutable materials such as polysilicon. Beyond static spatial temperature gradients, time variation in temperature is a universal phenomenon in the use of electronic circuits and systems, occurring every time an electronic system is turned on (powered up), and occurring continually as the ambient environment around the circuit changes. Analog chip and system designers devote considerable effort to ensuring robustness in the presence of temperature changes. This is because, in general, the properties of all materials exhibit some changes with temperature. A primary example is that of temperature coefficient of resistance (TCR).
In the calibration of a high precision circuit, it is advantageous to have fine-adjustment control simultaneously over both circuit component parameter values (such as resistance) and their temperature coefficients (such as TCR). Fine adjustment of resistance, while TCR changes in a less-easily-measured manner, is problematic, since the circuit is liable to lose its calibration any time the temperature varies from the temperature at which calibration was executed. This non-ideally under conditions of external temperature fluctuation would become more and more severe for higher precision of adjustment. Indeed the problem of simultaneous control of both resistance and TCR is a great source of difficulty in the analog electronics industry. Because of this, the measurement and control of TCR of resistance elements is very important for high-precision circuits. This is particularly important when one considers circuits and systems which are adjustable to high-precision. The higher the precision of adjustment of an adjustable element (or of an overall circuit or system), the smaller the temperature, variation which can significantly change the calibration of the circuit or system.
For example, consider two resistors having resistance values matched within 10 ppm. If the relative TCR (RTCR) is mismatched by as little as 1 ppm/K, then the resistance will drift by 50 ppm over a 50° C. range, overwhelming the fine adjustment of resistance. This situation is not optimal, since in the calibration of a high precision circuit, one needs fine-adjustment control simultaneously over both resistance and TCR, in order to have the needed control over one's circuit. There is clearly a need for rapid measurement and adjustment of TCR, to high precision, to accompany high-precision adjustment of resistance.